A molecular and evolutionary study of the β-globin gene family of the Australian marsupial Sminthopsis crassicaudata

β-globin gene families in eutherians (placental mammals) consist of a set of four or more developmentally regulated genes which are closely linked and, in general, arranged in the order 5'-embryonic/fetal genes-adult genes- 3'. This cluster of genes is proposed to have arisen by tandem duplication of ancestral β-globin genes, with the first duplication occurring 200 to 155 MYBP just prior to a period in mammalian evolution when eutherians and marsupials diverged from a common ancestor. In this paper we trace the evolutionary history of the β-globin gene family back to the origins of these mammals by molecular characterization of the β-globin gene family of the Australian marsupial Sminthopsis crassicaudata. Using Southern and restriction analysis of total genomic DNA and bacteriophage clones of β- like globin genes, we provide evidence that just two functional β-like globin genes exist in this marsupial, including one embryonic-expressed gene (S,c-ε) and one adult-expressed gene (S,c-β), linked in the order 5'-ε- β-3'. The entire DNA sequence of the adult β-globin gene is reported and shown to be orthologous to the adult β-globin genes of the North American marsupial Didelphis virginiana and eutherian mammals. These results, together with results from a phylogenetic analysis of mammalian β-like globin genes, confirm the hypothesis that a two-gene cluster, containing an embryonic- and an adult-expressed β-like globin gene, existed in the most recent common ancester of marsupials and eutherians. Northern analysis of total RNA isolated from embryos and neonatals indicates that a switch from embryonic to adult gene expression occurs at the time of birth, coinciding with the transfer of the marsupial from a uterus to a pouch environment.